Profile “backgrounds” or “cover images” are becoming extremely common. Facebook has 'em, Twitter has 'em, MySpace had 'em all the way back in 1997 or whenever it was.
SSO is for integrating with a site. Some of those sites will have profile backgrounds or cover images. Ergo, SSO should provide the ability to override this alongside the ability to override avatars. I assume much of the implementation would be similar.